A conventional fluid catalytic cracking system generally includes a fluid catalytic cracking (FCC) unit coupled to a catalyst injection system, a petroleum feed stock source an exhaust system, and a distillation system. The FCC unit includes a regenerator and a reactor. The reactor primarily houses the catalytic cracking reaction of the petroleum feed stock and delivers the cracked product in vapor form to the distillation system. Spent catalyst from the cracking reaction is transferred from the reactor to the regenerator to regenerate the catalyst by removing coke and other materials. The regenerated catalyst is then reintroduced into the reactor to continue the petroleum cracking process. The catalyst injection system maintains a continuous or semi continuous addition of fresh catalyst to the inventory circulating between a regenerator and a reactor.
During the catalytic process, there is a dynamic balance of the total catalyst within the FCC unit. For example, catalyst is periodically added utilizing the catalyst injection system and some catalyst is lost in various ways such as through the distillation system, through the effluent exiting the regenerator, etc. If the amount of catalyst within the FCC unit diminishes over time, the performance and desired output of the FCC unit will diminish, and the FCC unit will become inoperable. Conversely, if the catalyst inventory in the FCC unit increases over time or becomes deactivated, the catalyst bed level within the regenerator reaches an upper operating limit and the deactivated or excess catalyst is withdrawn to prevent unacceptably high catalyst emissions into the flue gas stream, or other process upsets. Thus, the typical fluid catalytic cracking system also contains a withdrawal apparatus suitable for withdrawing materials from one or more units, like FCC units.
U.S. Pat. No. 7,431,894 teaches a catalyst withdrawal apparatus and method for regulating catalyst inventory in a fluid catalytic cracking catalyst (FCC) unit. In this design, a heat dissipater is located adjacent the metering device and is adapted to cool catalyst entering the pressure vessel.
U.S. Pat. No. 8,092,756 teaches a catalyst withdrawal apparatus and method for regulating catalyst inventory in a unit. One embodiment of this catalyst withdrawal apparatus includes a vessel coupled to a heat exchanger.
U.S. Pat. No. 8,146,414 teaches a method comprising withdrawing material from a FCC unit to a heat exchanger coupled to the fluid catalytic cracking unit. The heat exchanger has a material inlet; a material outlet; a cooling fluid inlet and a cooling fluid outlet with respective temperatures. The method further comprises measuring the respective temperatures at the material inlet, material outlet, cooling fluid inlet and cooling fluid outlet of the heat exchanger; determining a change in temperature between the material inlet and material outlet and determining a change in temperature between the cooling fluid inlet and cooling fluid outlet; and correlating the change in temperature between the material inlet and material outlet and the change in temperature between the cooling fluid inlet and cooling fluid outlet to a metric of material withdrawn from the unit.
U.S. Pat. No. 8,146,414 further teaches that the cooled material may be moved to a collection vessel. The inventors have found that since the flow of catalyst in a typical withdrawal process is continuous until the collection vessel is full, the withdrawal process from the FCC unit has to be temporarily stopped once the collection vessel is full so that pressure can be applied to the storage vessel, in order to blow the catalyst out of it into the main refinery storage silo. Once the collection vessel is empty, the withdrawal process can begin again. The inventors have found that the heat exchange equipment operates at a steady (hot) temperature for several hours (as long as it takes to fill the collection vessel), followed by a short period when there is no source of heat, and so it will naturally cool down. After this, when withdrawal starts again the exchangers heat up and stay at high temperature for as long as it takes the collection vessel to fill up. This means that the exchanger skid experiences a “thermal cycle” of contraction and expansion every time the collection vessel fills up and needs to be emptied. For a system withdrawing 10 tons/day using a 5 ton collection vessel, this will result in two complete thermal cycles every day. Higher rates will result in even more frequent cycling. Every thermal cycle stresses the piping and pipe supports as the piping expands and contracts, and increases the likelihood of cracks developing, and subsequent weld failure. Similar problems are encountered with such a heat exchanger design when extremely cold streams need to be warmed, for example cryogenic liquids that need to be brought to room temperature or above. Even when the piping is designed with flexible supports to allow for the piping to expand and contract without creating excessive stresses, these supports have tendency to jam over time if not properly maintained. This increases the stresses and increases the likelihood of failure. There is therefore the need for a withdrawal system which reduces the number of temperature cycles which the heat exchanger portion is subjected to.
A further problem with thermal cycling occurs when certain alloys such as high carbon grades of stainless steel are used. It is well known that all H-grade austenitic stainless steels as well as some Fe—N—Cr alloys (alloy 800H/800HT, etc.) are susceptible to stress relaxation cracking (SRC) in the temperature range 550 to 750° C. The susceptible materials fail in a brittle manner and the cracks are always located in cold-formed areas or in welded joints. Most stress relaxation cracking failures occur within 1-year service. The major cause of relaxation cracking is lack of high temperature ductility. Many austenitic materials show an age hardening behaviour at temperatures between 500° C. and 750° C. Much of the piping on the inlet section of the heat exchanger section of the catalyst withdrawal system will operate within this temperature range, and will be subject to stress relation cracking.
The heat exchange portion of a catalyst withdrawal system is usually the most expensive portion of the system, as it has to be built using special metallurgy, using tightly controlled manufacturing techniques. It is therefore desirable to maximise the operational life of this equipment by reducing the cyclic stresses that is exposed to in normal operation.
Any invention which reduces the amount of thermal cycling of equipment operating in this critical temperature range is therefore highly desirable, as it will significantly improve operational reliability, decrease the risk of failure, and increase the level of safety of such a system.
It is therefore desirable to attain an improved withdrawal system for withdrawing particulate material from high temperature operations such as fluid catalytic cracking (“FCC”) process. We have discovered a new withdrawal system for withdrawing particulate material from industrial processes.